The design of devices that operate at very low temperatures including, for example, the proposed Superconducting Super Collider (SSC), has brought about the need for new solutions to the problem of providing adequate insulation for the components operating at low or cryogenic temperatures. The SSC is an advanced proton-proton collider for use in high energy physics research. The collider will consist of two accelerator rings, each 30 kilometers in diameter and housed in a common tunnel. The rings will accelerate protons to energies up to 20 trillion electron volts (TeV) prior to collision of the protons in particle detection facilities. In order to achieve these energies, the rings will incorporate superconducting magnets to bend the proton beam (dipole magnets) and to focus the beam (quadrupole magnets). The superconducting magnets will operate at cryogenic temperature, i.e., about 4.5 Kelvin (4.5K), and will be encased in cryostats, namely, vessels for maintaining a vacuum and constant low temperature. Approximately ten thousand cryostats will be connected end to end to form the SSC accelerator rings. The cryostats and their components must therefore not only be mechanically reliable and thermally stable, but also manufacturable at low cost.
The cryostats play a crucial role in the overall performance of the SSC and other similar devices operating at very low temperatures. The cryostats must minimize heat leak from the outside environment to the superconducting magnets in order to maintain the required cryogenic operating temperature. In fact, the ultimate operating cost of the SSC will depend principally upon the ability of the cryostats to prevent heat leak to the magnets.
The major components of the SSC cryostats are the cryogenic piping, the cold mass assembly (which includes the superconducting magnets), the insulation system, the vacuum vessel, the interconnections between cryostats, and the system for supporting or suspending the cold mass assembly. The insulation system must exhibit high impedance to heat leak from the outside environment to the cold mass assembly. In addition, the insulation system must exhibit dimensional stability over the expected twenty year operating life of the SSC, particularly in response to the numerous warm-ups and cool-downs that the SSC will undergo during its lifetime. The insulation system must also be inexpensive to manufacture and assemble, as well as easy to install, adjust and repair. Very similar concerns apply as well to insulation systems for other devices operating at low temperatures, regardless of the particular construction or tasks performed by such devices.
In each cryostat of the SSC, the cold mass assembly housing the superconducting magnets is surrounded by several regions of progressively higher temperature. The first region directly surrounding the cold mass assembly is the 4.5K region, cooled by cryogenic piping containing liquid helium at 4.35K. A second region known as the 20K region surrounds the 4.5K region. The 20K region is cooled by cryogenic piping containing gaseous helium at 20K. The 20K region is surrounded by a thermal shield (known as "the 20K shield") formed from aluminum sheet metal and around which insulation is wrapped. A third region known as the 80K region surrounds the 20K shield. The 80K region is cooled by cryogenic piping containing liquid nitrogen at 77K. Surrounding the 80K region is another thermal shield (known as "the 80K shield") formed from aluminum sheet metal and around which insulation is wrapped. A vacuum vessel at room temperature (300K) surrounds the 80K shield, and all the internal components of the cryostat are subjected to vacuum during operation of the collider.
As stated previously, the insulation installed around the thermal shields of the SSC cryostats must exhibit high impedance to heat leak as well as dimensional stability. To achieve the impedance to heat leak required in such cryogenic applications, insulation in the form of multilayer blankets of thermally reflective material has been found most effective. With respect to dimensional stability, the insulation must be able to withstand contraction in its length and width dimensions caused by exposure to temperature decreases from room temperature to as low as 4.5 Kelvin (4.5K). In addition, the insulation for the SSC will be fitted with openings through which the structures supporting or suspending the cold mass assembly will penetrate. Since the cryostat components, including the insulation, will be maintained in a vacuum, the surfaces of the insulation must also be kept substantially free of contaminants. The presence of such contaminants increases the amount of time and energy necessary to establish the vacuum.
Past techniques used in the preparation of multilayer fabrics could not produce multilayer insulation blankets meeting the requirements for applications like the SSC cryostat design. An 1895 patent to Palmer et al., U.S. Pat. No. 538,464, describes an early apparatus for measuring and cutting fabrics which includes a rotatable mandrel. A 1902 patent to Pope, U.S. Pat. No. 692,474, describes a large reel around which successive layers of paper are wound in layers of equal circumference. More recently, Pierson U.S. Pat. No. 2,208,774, issued in 1940, describes a mandrel apparatus used to produce large quantities of identical lengths of cloth. Each of these prior techniques involves the use of a rotatable mandrel to wind successive layers of material. However, these prior mandrels are all designed to contract in radius to maintain an equal circumference over which the material is wound. Consequently, use of these prior techniques would result in the production of multilayer assemblies having layers of identical length and width. Insulation blankets having layers of identical length and width would be unsuitable for cryogenic applications like the SSC, where the layers closest to the cryogenic structure experience more thermal contraction than those furthest away from the cryogenic structure.
Other past techniques would also be unsuitable for the mass production of multilayer insulation blankets for cryogenic applications like the SSC. Tolliver U.S. Pat. No. 4,201,351, issued in 1980, describes an apparatus including a cone-shaped mandrel for cutting plastic film into trapezoidal shapes of varying dimensions. Such a conically shaped mandrel, however, would cause individual layers of the thin, flexible insulating material to travel or "walk" toward the smaller diameter end of the mandrel while being wound, thereby disturbing the registration of the layers.
Other known techniques for producing multilayer assemblies often involve the stacking of successive layers by hand on a flat surface such as a workbench. Such manual stacking techniques suffer from difficulties in maintaining registration of the layers and uniform layer density. Layer registration refers to maintaining the layers in precise alignment in the length and width dimensions. Layer density is a measure of the number of layers per unit thickness, and is most often expressed as the number of layers thermally reflective material per centimeter. Manual stacking techniques are also more likely than mandrel-based winding techniques to introduce unwanted contaminants onto layer surfaces. In addition, manual stacking techniques are more labor intensive than mandrel-based winding techniques, thereby increasing the cost to mass produce the finished insulation blankets.
The present invention is directed to overcoming these and other difficulties inherent in prior multilayer insulation blankets and techniques for fabricating such insulation blankets. In the present invention, multilayer insulation blankets are produced from thermally reflective material. Each successive layer of thermally reflective material is slightly greater in length and width than the preceding layer as the layers are traversed in the direction of the cryogenic structure around which the blanket is installed. To accomplish such a dimensional gradient in the layer assembly, a rotatable mandrel having an outer surface of fixed radius and of convex cross-section is used to wind successive layers of thermally reflective material. The layers are bound together along two lines parallel to the edges of the circumference of the mandrel. The layers are then cut along a line parallel to the axle of the mandrel and removed from the mandrel, resulting in a multilayer blanket that is suitable for cryogenic applications like the SSC.